TGFbeta is a potent endogenous tumor suppressor and TGFbeta-induced growth arrest is abrogated in almost all human cancers. Strikingly, the Smad proteins thought to mediate TGFa-induced growth arrest are present and functional in most human mammary carcinomas. These observations suggest that in human breast cancer TGFa signaling is not inactivated per se, but that TGFbeta signaling becomes uncoupled from cell cycle regulation. We have discovered that rapamycin, an anti-cancer drug currently in clinical trials, cooperates with TGFa to induce growth arrest of normal mammary epithelial cells, and restores TGFbeta-induced cell cycle arrest in Myc, Ras, and E2F1 transformed epithelial cells. Rapamycin also restores TGFbeta-induced growth arrest in several human carcinoma cell lines. Rapamycin cooperates with TGFbetaa to inhibit cell proliferation, which results from inactivation of the cyclin-dependent kinase Cdk2. Inhibition of Cdk2 is associated with increased binding to the cyclin dependent kinase inhibitor p27. We hypothesize that rapamycin functions with TGFbeta to inhibit the proliferation of human mammary carcinoma cells by cooperatively inhibiting Cdk2 activity through increased association with p27, and through the dissociation of the E2F transcription factor from Cdk2. We further hypothesize that rapamycin cooperates with TGFbeta present in vivo to inhibit tumor growth. These hypotheses will be tested in the following Specific Aims: 1) Determine the mechanisms by which TGFbeta + rapamycin treatment induces growth arrest of no transformed mammary epithelial cells and human mammary carcinoma cells, 2) Determine the mechanisms by which TGFbeta and rapamycin regulate p27 function, and 3) Examine the interaction between TGFbeta and rapamycin anti-tumor activities in vivo. The proposed studies will employ breast cancer as a model system, but will have important implications for a wide array of tumor types. The results of these studies will yield important insights into the synergistic growth inhibitory mechanisms of TGFbeta and rapamycin, a potent endogenous tumor suppressor, and a drug currently in clinical trials against breast cancer, respectively.